Method and apparatus for managing actinic intensity transients in a lithography mirror

ABSTRACT

An apparatus and method of maintaining a time-constant heat load on a lithography mirror. The mirror includes a resistive layer formed on a substrate, contacts for coupling a power supply to the resistive layer, an insulating sublayer formed on the resistive layer, a polished layer formed on the insulating layer, and a reflective layer formed on the polished layer. The time-constant heat load on the lithography mirror is maintained by placing an additional electrical heat load on the mirror according to the actinic heat load transmitted by the mask. Maintaining the time-constant heat load can reduce or eliminate variation in image distortion that occurs as a result of changes in actinic heat load on the lithography mirror. Independent temperature control can be used to mitigate “cold-edge effect.”

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to lithography systems.More particularly, the present invention relates to management ofactinic heat load on mirrors in lithography systems.

[0003] 2. Background Art

[0004] Lithography is a process used to create features on the surfaceof substrates. Such substrates can include those used in the manufactureof flat panel displays, circuit boards, various integrated circuits, andthe like. A frequently used substrate for such applications is asemiconductor wafer. One skilled in the relevant art would recognizethat the description herein would also apply to other types ofsubstrates.

[0005] During lithography, a wafer, which is disposed on a wafer stage,is exposed to an image projected onto the surface of the wafer by anexposure system located within a lithography system. The exposure systemincludes a reticle (also called a mask) for projecting the image ontothe wafer.

[0006] The reticle is generally located between a semiconductor chip anda light source. In photolithography, the reticle is used as a photo maskfor printing a circuit on a semiconductor chip, for example. Lithographylight shines through the mask and then through a series of opticallenses that shrink the image. This small image is then projected ontothe silicon or semiconductor wafer. The process is similar to how acamera bends light to form an image on film. The light plays an integralrole in the lithographic process. For example, in the manufacture ofmicroprocessors (also known as computer chips), the key to creating morepowerful microprocessors is the size of the light's wavelength. Theshorter the wavelength, the more transistors can be etched onto thesilicon wafer. A silicon wafer with many transistors results in a morepowerful, faster microprocessor.

[0007] As chip manufacturers have been able to use shorter wavelengthsof light, they have encountered a problem of the shorter wavelengthlight becoming absorbed by the glass lenses that are intended to focusthe light. Due to the absorption of the shorter wavelength light, thelight fails to reach the silicon wafer. As a result, no circuit patternis created on the silicon wafer. In an attempt to overcome this problem,chip manufacturers developed a lithography process known as ExtremeUltraviolet Lithography (EUVL). In this process, a glass lens can bereplaced by a mirror. Although the mirror reflects a large percentage ofthe light, a fair amount of the light is absorbed by the mirror. Theabsorbed actinic light (i.e., energy generated from a light source suchas an optical light source in a lithography tool) causes heat load onthe mirror. Too much heat can result in image distortion on the wafer.Further, if heat load on the mirror is not maintained at a relativelyconstant level, variation in the amount of image distortion can occur.Thus, there is a need to control actinic heat load (e.g., by measuringmirror temperature) on the mirror caused by the absorbed light.

[0008] The temperature of the mirror should be controlled such that thetemperature is maintained constant over time. Conventional mirrortemperature control techniques attempt to maintain a time-constantmirror temperature by varying the rate of heat removal from thenon-optical surfaces of the mirror with a temperature servo. A typicalmirror is relatively large and has a high thermal mass with low thermalconductivity. Due to the two above mentioned characteristics of thetypical mirror in a lithography projection system, this conventional“control-by-heat-removal” method can be ineffective in environments withtransient actinic heat loads. For example, in applications such as EUVphotolithography of integrated circuits, the actinic heat load istransient (e.g., changes every time a reticle is exchanged). The actinicheat load changes faster than the temperature control servo's ability tofollow. As a result, the temperature of the mirror is not maintained ata constant over time and variation in distortion of the projected imageoccurs.

[0009] The problem of image distortion variation resulting from failureto maintain a time-constant and spatially-constant heat load on themirror is further exacerbated by a phenomenon known as “the cold edgeeffect.” The cold edge effect is caused by the variation of actinic heatload on the optical aperture of the mirror and the annular area (i.e.,the non-illuminated area of the mirror located beyond the opticalaperture. A lithography mirror typically has a lower temperature at theannular area than it has at the optical aperture.

[0010] Therefore, what is needed is an apparatus and method forfabricating a mirror and for managing heat load on the mirror such thatvariation in image distortion from variation of heat on the mirror isminimized. Such an apparatus and method should maintain a time-constanttotal heat load during transients of illumination incident on theprojection mirror (i.e., during times of change of actinic heat load onthe mirror). Further, such an apparatus and method should also maintaina spatially constant total heat load on the mirror to mitigate the coldedge effect.

BRIEF SUMMARY OF THE INVENTION

[0011] The present invention comprises an apparatus and method formaintaining a time-constant total heat load on a lithography mirror. Thelithography mirror includes a resistive layer formed on a substrate,contacts for coupling a power supply to the resistive layer, aninsulating layer formed on the resistive layer, a polished layer formedon the insulating layer, and a reflective layer formed on the polishedlayer. The time-constant heat load on the lithography mirror ismaintained by placing an additional heat load on the mirror as needed. Adesired time-constant heat load on the lithography mirror is determinedbased on the amount of actinic heat reflected onto the mirror by one ofthe most reflective masks with a heat reflection capability unsurpassedby any of the other masks in a set of masks. When one of the lessreflective masks is being used, additional heat is applied to the mirrorto achieve the desired time-constant heat load on the mirror.

[0012] Maintaining the desired time-constant heat load on the mirror canreduce or eliminate variation in image distortion that occurs as aresult of changes in actinic heat load on the lithography mirror. Tomitigate the cold edge effect, the mirror can be divided into one ormore zones with independent temperature control. This allows a constantadditional heat load to be applied to a first zone while allowing thetemperature in a second zone to be inversely modulated according toactinic heat load on the projection mirror. The flexibility ofindependent temperature control also allows a spatially constant totalheat load.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0013] The accompanying drawings, which are incorporated herein and formpart of the specification, illustrate the present invention and togetherwith the description further serve to explain the principles of theinvention and to enable a person skilled in the relevant art(s) to makeand use the invention.

[0014]FIG. 1 is an illustration of layers and components of alithography mirror according to the present invention.

[0015]FIG. 2A is a side view of layers of a lithography mirror accordingto the present invention.

[0016]FIG. 2B is an illustration of the wiring layer of FIG. 2A.

[0017]FIG. 2C is a top view of layers and components of the lithographymirror depicted in FIG. 1.

[0018]FIG. 3 is a flow diagram of the steps involved in manufacturing alithography mirror according to the present invention.

[0019]FIG. 4 is an illustration of a lithography illumination system inwhich the lithography mirror of the present invention is shown as acondenser mirror.

[0020]FIG. 5 is an illustration of a lithography projection system inwhich the lithography mirror of the present invention is shown as aprojection optics mirror.

[0021]FIG. 6 is a flow diagram of the steps involved in maintaining atime-constant heat load on a lithography mirror according to the presentinvention.

[0022]FIG. 7A is an illustration of a lithography mirror depicting adivision of the mirror into zones.

[0023]FIG. 7B is an illustration of a top view of the lithography mirrorof FIG. 7A.

[0024]FIG. 7C is an illustration of a wiring layer of the annular zoneof the lithography mirror of FIG. 7A.

DETAILED DESCRIPTION OF THE INVENTION

[0025]FIG. 1 illustrates a cross-sectional view of various layers andcomponents of a lithography mirror 100 according to the presentinvention.

[0026] Lithography mirror 100 is composed of a mirror blank substrate105, a resistive layer 107, a front edge 109, a rear edge 111, contacts113 (e.g., electrodes) formed on resistive layer 107 for coupling apower supply (shown in FIG. 4) to resistive layer 107, a wiring layer115, a polished layer 117, and a reflective layer 119.

[0027] Mirror blank substrate 105 is typically made of glass (e.g., lowexpansion glass, silicon, or quartz) and has a diameter-to-thicknessratio of approximately three to five. Mirror blank substrate 105represents the basic structure of lithography mirror 100. The substrateshould be machined and polished and have a near-zero Coefficient ofThermal Expansion (CTE), in accordance with standard industry practices.CTE is a thermodynamics term used to refer to the amount of increase insize of a solid object that occurs because of an increase intemperature. The term is well known to those skilled in the relevantart(s) and will therefore not be described further herein.

[0028] Resistive layer 107 is the first layer of lithography mirror 100.Resistive layer 107 can be an electrically resistive layer or film. Itshould be noted that resistive layer 107 should be applied to an activeside of lithography mirror 100 (i.e., the reflective side of lithographymirror 100). Resistive layer 107 dissipates power as heat in lithographymirror 100. Resistive layer 107 can have a low, medium, or highresistance. Depending upon a desired value of resistance, resistivelayer 107 can be made of Carbon, Nichrome, some mixture of ceramic andmetal (cermet), or any other viable material(s) known to those skilledin the relevant art(s).

[0029] To vary electrical conductivity in resistive layer 107, itsthickness can be varied to allow the center of the film to produce moreheat than the periphery of the film. This type of variation can providean optimal accommodation for the distribution of incident actinic poweror heat within the optical aperture of lithography mirror 100.

[0030] Resistive layer 107 can be produced by doping a semiconductorfilm (e.g., arsenic-doped silicon) to vary its electrical conductivity.If resistive layer 107 is doped, the amount of dopant concentration canbe varied to allow the center of resistive layer 107 to produce moreheat than the periphery. The variation of the amount of dopantconcentration on resistive layer 107 can allow optimal accommodation forthe distribution of incident actinic power within the optical apertureof lithography mirror 100.

[0031] Wiring layer 115 is the second layer of lithography mirror 100.Wiring layer 115 includes contacts 113 and insulating sublayer 114.Wiring layer 115 will be further described in FIG. 2B.

[0032] Polished layer 117 is the third layer of lithography mirror 100.Polished layer 117 should be polished to final figure, as is well knownto those skilled in the relevant art(s). For example, the deviationbetween the actual polished surface achieved in the mirror compared tothe ideal polished surface of the mirror should be less than onenanometer. Polished layer 117 is made of any viable polishable materialknown to those skilled in the relevant art(s). It should be noted thatif polished layer 117 is composed of conductive material, an insulatinglayer should be added between wiring layer 115 and polished layer 117 toprevent short-circuiting. Alternatively, the insulating layer itself canbe polished and substituted for the polished layer.

[0033] Reflective layer 119 is the fourth layer of lithography mirror100. Reflective layer 119 provides lithography mirror 100 of the presentinvention with its reflectiveness characteristic. Reflective layer 119is made of any viable material known to those skilled in the relevantart(s) for fabricating lithography mirrors. For example, in an EUVmirror, reflective layer 119 can be made of a Moly-silicon multilayer.

[0034] It should be noted that thickness of the various layers oflithography mirror 100 of the present invention has been exaggerated forillustrative purposes. The actual thickness of each layer can vary fromless than one micron to a few microns. For example, the thickness ofreflective layer 119 can be a fraction of a micron.

[0035]FIG. 2A illustrates a side view of the layers and components oflithography mirror 100 (shown in FIG. 1).

[0036]FIG. 2B illustrates wiring layer 115 of lithography mirror 100.Wiring layer 115 comprises contacts 113 a and 113 b (generallydesignated as contacts 113) and insulating sublayer 114. Contacts 113(e.g., electrodes or other equivalents) are made of copper or any othersuitable conductive material. Contacts 113 couple to a power supply viawires 220 a and 220 b. This allows the power supply to disperse heat inresistive layer 107 (which lies below wiring layer 115). Contacts 113should be spaced such that they provide optimal uniform dispersion ofheat in lithography mirror 100. For example, the contact(s) can bediametrically opposed to one another.

[0037] Insulating sublayer 114 covers resistive layer 107. Insulatingsublayer 114 is made of a dielectric material that reduces thepossibility of the occurrence of short circuits in lithography mirror100 of the present invention. For example, insulating sublayer 114 canbe a nonconductive material such as polymer. Insulating sublayer 114 canalso be made of silicon dioxide or any other viable insulatingmaterial(s) known to those skilled in the relevant art(s) forinsulating. Insulating sublayer 114 should be of approximately the samethickness (e.g., less than one micron) as contacts 113 to allow wiringlayer 115 to be relatively flat.

[0038]FIG. 2C illustrates a top view of the layers and components oflithography mirror 100.

[0039]FIG. 3 is a flow diagram 300 illustrating the steps involved inmanufacturing a lithography mirror according to the present invention.The process begins with step 305, and immediately proceeds to step 310.

[0040] In step 310, resistive layer 107 is formed on the opticallyactive side (i.e., the reflective side) of mirror blank substrate 105.As described above, resistive layer 107 can be made of any materialsuitable for providing electrical resistance.

[0041] In step 315, one or more contacts such as electrodes are coupledto an edge(s) of resistive layer 107.

[0042] In step 320, insulating layer 115 is formed on resistive layer107 to reduce the possibility of the occurrence of short circuits inlithography mirror 100.

[0043] In step 325, a layer of polished material is formed on insulatinglayer 115 to form polished layer 117 (shown in FIG. 1). As mentionedabove, polished layer 117 can be made of any viable polished materialknown to those skilled in the relevant art(s).

[0044] In step 350, reflective layer 119 is formed on polished layer117. Reflective layer 119 provides lithography mirror 100 with itsreflectiveness characteristic.

[0045]FIG. 4 illustrates a lithography illumination system 400 depictinglithography mirror 100 acting as a lithography condensor mirror. FIG. 4illustrates a situation in which an actinic light source is varied. Forexample, a lithography tool user can increase or decrease the actiniclight intensity being produced by the actinic light source in such asituation. Lithography illumination system 400 represents theilluminator portion of a lithography tool (i.e., transmission of EUVlight before it is projected onto the reticle or mask stage).

[0046] Lithography illumination system 400 comprises collector mirror420, EUV light source 415, lithography mirror 100, mirror temperaturesensor 445, actinic light intensity sensor 425, power adjusting circuit430, and variable power supply 450. Collector mirror 420 reflects lightfrom EUV light source 415 onto lithography mirror 100. EUV light source415 can be a three-dimensional beam of light reflected from collectormirror 420 to lithography mirror 100.

[0047] A light beam 410 a is a beam of actinic light being transmittedfrom EUV light source 415 to lithography mirror 100. Light beam 410 b isa beam of actinic light reflected from lithography mirror 100 to areticle stage (not shown in FIG. 4), for example.

[0048] In FIG. 4, lithography mirror 100 acts as a condenser mirror. Itsfundamental operation (i.e., how it reflects light) is well known tothose skilled in the relevant art(s). Lithography mirror 100 is composedof the same layers as indicated in FIG. 1. In addition to the layerspreviously described, however, lithography mirror 100 further comprisesmirror temperature sensor 445.

[0049] Mirror temperature sensor 445 measures the temperature inlithography projection mirror 100. Mirror temperature sensor 445 acts asa feedback means, thereby transmitting a voltage signal that is roughlyproportional to the temperature of the sensor itself to power adjustingcircuit 430 to cause power applied to lithography mirror 100 to beincreased or decreased. For example, mirror temperature sensor 445 canbe at least one infrared detector to monitor a front surface oflithography projection mirror 100.

[0050] Mirror temperature sensor 445 can also be a thermocouple that isattached to the front surface of lithography mirror 100. The termthermocouple is used herein to refer to a single thermocouple, athermistor, a resistive temperature detector, or any combination ofthese elements. Mirror temperature sensor 445 should be positioned so asto not impede reflection of actinic light beam 410 a. Mirror temperaturesensor 445 should also be placed as close as possible to a positionwhere the light impinges on and heats lithography mirror 100. Further,those skilled in the relevant art(s) would recognize that any other typeof sensor or detector (or combination thereof) can be employed withoutdeparting from the spirit and scope of the present invention.

[0051] Mirror temperature sensor 445 can be coupled to power adjustingcircuit 430 via an energy channel 435 a. Energy channel 435 a can be anyviable channel for transferring electricity. A first end of energychannel 435 a connects to power adjusting circuit 430. A second end ofenergy channel 435 a connects to mirror temperature sensor 445 onlithography mirror 100, as illustrated in FIG. 4.

[0052] Actinic light intensity sensor 425 measures actinic lightreflected from collector mirror 420 onto lithography mirror 100. Actiniclight intensity sensor 425 acts as a feed forward means, therebyproviding a signal to power adjusting circuit 430 which causes variablepower supply 450 to increase or decrease electrical power to theresistive film in the mirror inversely to the amount of reflectedactinic light sensed. Actinic light intensity sensor 425 can be composedof a heat flux sensor or a photocell capable of producing an electricalvoltage that is proportional to the intensity of the light incident onthe heat flux sensor or photocell.

[0053] Actinic light intensity sensor 425 is also coupled to poweradjusting circuit 430 via an energy channel 435 b, such as a conductivewire. For example, a first end of energy channel 435 b connects to poweradjusting circuit 430. A second end of energy channel 435 b connects toactinic light intensity sensor 425.

[0054] Although actinic light intensity sensor 425 can be employed incombination with mirror temperature sensor 445, either one or the othercan be utilized alone. Employing both actinic light intensity sensor 425and mirror temperature sensor 445, however, can improve performance oflithography illumination system 400 by providing a more accurate overallmeasurement.

[0055] Power adjusting circuit 430 reacts to input signals from mirrortemperature sensor 445 and actinic light intensity sensor 425 bychanging a command signal to variable power supply 450. For example,when power adjusting circuit 430 detects a change in the signal fromactinic light intensity sensor 425 or mirror temperature sensor 445, itcommands variable power supply 450 to inversely change the power sent tocontacts 113 on resistive layer 107. Power adjusting circuit 430 can belocated in a remote electronics cabinet of lithography illuminationsystem 400.

[0056] Variable power supply 450 provides power to contacts 113, therebydissipating heat in resistive layer 107 (shown in FIG. 1). Variablepower supply 450 can act as a variable resistor to vary the amount ofelectrically generated heat load dissipated in resistive layer 107 inlithography mirror 100 inversely to the amount of actinic heat load onlithography mirror 100, as measured by actinic light intensity sensor425, according to the following equation:

TH=C=AH+EH,

[0057] wherein TH is total heat load on the lithography mirror,

[0058] C is some time-constant power,

[0059] AH is actinic heat load on the lithography mirror, and

[0060] EH is electrical heat load on the lithography mirror.

[0061] Therefore, the amount of electrical heat load needed to maintaina time-constant temperature can be represented by the followingequation: EH=C−AH. When an appropriate amount of electrical heat load isinversely applied (by variable power supply 450) to lithography mirror100 according to actinic light reflected onto the mirror (as measured byactinic light intensity sensor 425), variation in image distortion inlithography illumination system 400 can be reduced or eliminated.

[0062] Variable power supply 450 can be coupled to power adjustingcircuit 430 via an energy channel 432, as would be known to one skilledin the relevant art(s). Variable power supply 450 can be coupled tocontacts 113 of lithography mirror 100 via an energy channel 440,similar to the connection described above. Variable power supply 450 canbe a Direct Current (DC) or an Alternating Current (AC) type of powersupply, as would be known to those skilled in the relevant art. Variablepower supply 450 can be located in a remote electronics cabinet oflithography illumination system 400.

[0063]FIG. 5 is an illustration of a lithography projection opticssystem 500 depicting lithography mirror 100 acting as a lithographyprojection optics mirror. FIG. 5 illustrates a situation in which anactinic light source remains the same. In FIG. 5, however, changing ofmasks with various reflectivity capabilities causes a variation inactinic light reflected onto lithography mirror 100. Lithographyprojection optics system 500 represents the projection optics portion ofa lithography tool (i.e., after reflection of EUV light onto the reticleor mask stage).

[0064] Lithography projection optics system 500 comprises mask stage505, mask 507, illuminator mirror 510, EUV light source 515, actiniclight beams 520 a and 520 b, lithography mirror 100, actinic lightintensity sensor 525, power adjusting circuit 430, mirror temperaturesensor 445, and variable power supply 450.

[0065] Mask stage 505 is a standard mask stage used in a lithographyprojection tool, as would be known to one skilled in the relevantart(s). Mask stage 505 holds mask 507, which is used to etch an imageonto a wafer. Illuminator mirror 510 reflects actinic light beam 520 afrom EUV light source 515. Illuminator mirror 510 comprises collectorand condensor mirrors, for example. These devices are well known in therelevant art(s) and will not be described further herein.

[0066] Actinic light beam 520 a is an actinic beam of light reflectedfrom illuminator mirror 510 to mask stage 505. Actinic light beam 520 bis an actinic beam of light reflected from mask stage 505 to lithographymirror 100, as would be apparent to one skilled in the relevant art(s).

[0067] Actinic light intensity sensor 525 operates similar to actiniclight intensity sensor 425 (shown in FIG. 4). But unlike actinic lightintensity sensor 425, actinic light intensity sensor 525 can be rotated,as will be described below. As mentioned above, EUV light source 515remains constant (i.e., transmits an amount of actinic light thatremains constant over time). When actinic light beam 520 a is reflectedby mask 507 on mask stage 505, however, a change in actinic lightintensity reflected onto lithography mirror 100 occurs. For example, theactinic light intensity of actinic light beam 520 b varies depending onthe overall reflectivity of the mask being exposed at a particular time,as will be further described in a subsequent figure. Thus, at time t₁,actinic light intensity of actinic light beam 520 b has a first value asdetermined by reflectivity of the particular mask being used. At timet₂, actinic light intensity of actinic light beam 520 b can have a valuedifferent from the first value at time t₁, as determined by reflectivityof the particular mask being used at this time period.

[0068] It should be noted that actinic light intensity sensor 525 canencroach on actinic light beam 520 b as it is being reflected from maskstage 505. As a result, actinic light intensity sensor 525 can obscure aportion of a wafer (not shown) being lithographically printed.Measurement performed by actinic light intensity sensor 525 must,therefore, occur before wafer exposure. Thus, before wafer exposure,actinic light intensity sensor 525 is rotated to position 526 to obtaina measurement from actinic light beam 520 b. During exposure, however,actinic light intensity sensor is in position 527 to preventinterference with actinic light beam 520 b.

[0069]FIG. 6 is a flow diagram 600 illustrating the steps involved inmaintaining a time-constant total heat load on lithography mirror 100.Control begins with step 605 and proceeds immediately to step 610. As isknown to those skilled in the relevant art(s), in a typical lithographyimaging session, a plurality of masks having various reflectivecapabilities (i.e., heat transmission capabilities) can be used. Thus, afirst mask can be capable of reflecting three watts of power while asecond mask can be capable of reflecting only two watts of power, forexample.

[0070] In step 610, a determination is made of the amount of actinicheat transmitted by one of the most reflective masks on lithographymirror 100 (shown in FIG. 1). One of the most reflective masks is a maskwith a heat transmission capability unsurpassed by any other mask in theplurality of masks. For example, there can be four masks with a heatgeneration capability of five watts of actinic power. If no other maskin the plurality of masks has a heat generation capability that exceedsfive watts of actinic power, each of the four masks is considered to beone of the most reflective masks. It should be noted that no additionalheat (e.g., electrical heat from variable power supply 450) needs to beapplied when one of the most reflective masks is being used. This maskrepresents the “worst case scenario” and acts as a measurement baselinefor the amount of electrical heat needed when other masks are used.

[0071] In other words, the actinic heat load induced on lithographymirror 100 by one of the most reflective masks represents the desiredtime-constant heat load on lithography mirror 100 over time. The actinicheat transmitted by the other masks (i.e., the less reflective masks) isless than that transmitted by one of the most reflective masks. Tomaintain the desired time-constant heat load on lithography mirror 100,the heat load on the lithography mirror must be increased during use ofthe less reflective masks, by adding electrical power to the mirror.

[0072] In step 615, the amount of actinic heat transmitted by thecurrent mask is determined.

[0073] In decision step 620, it is determined whether actinic heattransmitted by the current mask is capable of generating an amount ofactinic heat equal to the actinic heat transmitted by one of the mostreflective masks.

[0074] It should be noted that no accounting is made for the situationin which the actinic heat transmitted by the current mask is greaterthan one of the most reflective masks. Such a situation should not occurbecause one of the most reflective masks generates an amount of heatthat is unsurpassed by any other mask in the plurality of masks.

[0075] Decision step 620 is needed to determine how much additional heat(e.g., electrical heat) needs to be applied to lithography mirror 100 tomaintain the desired time-constant heat load during use of the currentmask. As described above, feedforward means can be added to thelithography system to measure actinic heat transmitted by the currentmask.

[0076] In decision step 620, if actinic heat from the current mask isequal to the actinic heat capable of being produced by one of the mostreflective masks, then the current mask is one of the most reflectivemasks. Thus, no electrical heat needs to be applied to lithographymirror 100. In this situation, control returns to step 615, where anamount of actinic heat transmitted by the next mask is determined.

[0077] Alternatively, in decision step 620, if the current mask is notone of the most reflective masks, control resumes with step 625. In step625, electrical heat is applied to lithography mirror 100 to achieve aheat load on the mirror that is slightly less (e.g., 90-95% of the valuecalculated from the actinic light power measured in the feedforwardloop) than the heat load on the mirror during use of one of the mostreflective masks. Although the mirror should be warmed as quickly aspossible, recovering from a condition in which the mirror is warmedbeyond “most reflective mask heat load” can cause a longer delay thancould occur warming the mirror to allow its heat load to approach mostreflective mask heat load. The exact difference between the heat loadplaced on the mirror and the heat load placed on the mirror during useof the most reflective mask depends on how accurately the heat fluxsensor can measure the actinic heat input.

[0078] It should be noted that after accuracy of the system isdetermined (e.g., measurement, stability, and calibration), moreaggressive controls can be adopted (e.g., 98-99% of the value calculatedfrom the actinic light power measured in the feedforward loop).

[0079] For example, one of the most reflective masks can be capable ofreflecting approximately three watts of power on lithography mirror 100.Thus, it would be desirable to maintain a time-constant total heat loadof slightly less than three watts of power on lithography mirror 100.One of a set of less reflective masks can be capable of reflecting onlyone watt of power on lithography mirror 100. Thus, when using thisparticular mask, approximately two watts of electrical power would haveto be added to lithography mirror 100 to maintain the desiredtime-constant total heat load on the mirror.

[0080] In embodiments, a calibration mask can be utilized to improveaccuracy of the system, as would be apparent to those skilled in therelevant art(s). For example, the calibration mask would have thereflectivity of the most reflective mask. The entire active region (areaof the mask normally occupied by the pattern to be transferred to thewafer with a production mask) of the calibration mask can be coated withreflective coating. The active area can then be sized to correspond tothe largest active area specified for production masks to be run in thelithography tool (e.g., 108 mm x 136 mm). The coating and sizing canassist in ensuring that the amount of light reflected by the calibrationmask is equal to or exceeds the reflected light output of anyconceivable production masks that could be run in the tool. Thus, inthese embodiments, the calibration mask is the most reflective mask orone of the most reflective masks.

[0081] Determining the potential heat load transmitted by one of themost reflective masks and applying electrical heat accordingly whenusing one of the less reflective masks can reduce or eliminate variationin distortion of projected images. Thus, the need of having to coollithography mirror 100 by a variable cooling means can be eliminated.

[0082] Steps 630-640 show a proportional temperature control looptechnique. It would be apparent to those skilled in the art that a moresophisticated control loop could have been chosen. For example, aProportional Integral Derivative (PID) control loop could beimplemented.

[0083] In step 630, the temperature of the lithography mirror ismeasured. For example, this measurement can be performed by mirrortemperature sensor 445. This step is performed to determine if moreelectrical heat needs to be applied to lithography mirror 100 to obtainor maintain the desired time-constant temperature of lithography mirror100. For example, the Heaterstat™ control method (implemented by MincoProducts, Inc. of Minneapolis, Minn.) can be used to maintain thedesired time-constant temperature of lithography mirror 100. It shouldbe noted that measuring the temperature of lithography mirror 100 alsoguards against the possibility of applying too much heat to the mirrorsuch that the mirror's total heat load is greater than the desiredtime-constant heat load.

[0084] In step 635, the temperature of the lithography mirror iscompared to a setpoint. If the mirror temperature is greater than orequal to the setpoint, control resumes with step 630, where thetemperature of the mirror is measured.

[0085] Alternatively, in step 635, if the temperature is less than thesetpoint, additional heat (e.g., electrical heat) is applied to thelithography mirror in step 640.

[0086] In step 645, it is determined whether the switch-off signal hasbeen received. If the switch-off signal has not been received, controlresumes with step 630, where the mirror temperature is again measured.Alternatively, in step 645, if it is determined that the switch-offsignal has been received, operation of the system ceases, and controlends with step 650.

[0087]FIG. 7A is an illustration of a lithography mirror 700 depicting adivision of the mirror into independently controlled zones. Toaccommodate for lack of actinic heat in a first zone, additional heat(e.g., electrical heat) can be constantly applied to the first zonewhile simultaneously applied in a second zone by inversely modulatingheat. Lithography mirror 700 comprises reflective layer 119 (shown inFIG. 1), polished layer 117, optical aperture zone 702, annular zone703, and substrate 105.

[0088] Optical aperture zone 702 is the portion of lithography mirror700 which receives and reflects light. Additional heat in opticalaperture zone 705 can be independently inversely modulated according tothe actinic heat load on lithography mirror 700 to reduce or eliminatevariation in distortion, as described above. Optical aperture zone 702can comprise resistive layer 107 and wiring layer 115. It should benoted that wiring layer 115 of optical aperture zone 702 is the same asthat of wiring layer 115 shown in FIG. 2B.

[0089] Annular zone 703 is the portion of lithography mirror 700 whichreceives little or no actinic heat during operation of a lithographyprojection tool (known as the cold edge effect). As a result, annularzone 703 is typically cooler than optical aperture zone 702. Toaccommodate for its lack of actinic heat, additional heat can beconstantly applied to annular zone 703.

[0090] Annular zone 703 comprises resistive layer 710 and wiring layer715. Resistive layer 710 is the equivalent of resistive layer 107.Wiring layer 715 can couple to a power supply to provide heat to annularzone 703 of lithography mirror 700, as will be described incorresponding text of FIG. 7C.

[0091]FIG. 7B illustrates a top view of lithography mirror 700 of FIG.7A.

[0092]FIG. 7C illustrates wiring layer 715 of annular zone 703 of FIG.7A. Wiring layer 715 comprises contacts 750 a and 750 b, and insulatinglayer 755. Contacts 750 function in the same manner as contacts 113(shown in FIG. 1). It should be noted, however, that in wiring layer 715of annular zone 703, contacts 750 should be positioned concentricallysuch that flow of electricity between first contact 750 a and secondcontact 750 b produces a relatively annular and uniform heating pattern.Further, it should be noted that insulating layer 755 should be of thesame thickness as contacts 750 to allow wiring layer 715 to berelatively flat. Insulating layer 755 functions in the same manner asinsulating layer 114 (shown in FIG. 2B). Energy channels 745 (e.g.,electrical wires) can be used to connect contacts 750 to an additionalpower supply (i.e., another power supply in addition to the oneconnected to wiring layer 115) located in a remote electronics cabinetin a lithography tool, as would be apparent to those skilled in therelevant art(s).

CONCLUSION

[0093] While various embodiments of the present invention have beendescribed above, it should be understood that they have been presentedby way of example, and not limitation. It will be apparent to personsskilled in the pertinent art that various changes in form and detail canbe made therein without departing from the spirit and scope of theinvention. Thus, the present invention should not be limited by any ofthe above-described exemplary embodiments, but should be defined only inaccordance with the following claims and their equivalents.

What is claimed is:
 1. A lithography mirror for use in a lithographysystem that enables management of actinic heat load, comprising: asubstrate; a resistive layer formed on said substrate; a wiring layerformed on said resistive layer, wherein said wiring layer includes aninsulating sublayer and contacts for coupling to a power supply; apolished layer formed on said insulating sublayer; and a reflectivelayer formed on said polished layer.
 2. The lithography mirror of claim1, wherein said polished layer is formed on said insulating sublayer andsaid contacts.
 3. The lithography mirror of claim 1, wherein said powersupply is a variable power supply.
 4. The lithography mirror of claim 1,wherein said insulating sublayer comprises a polymer.
 5. The lithographymirror of claim 1, wherein said insulating sublayer comprises anonconductive material.
 6. The lithography mirror of claim 1, whereinsaid insulating sublayer comprises silicon dioxide.
 7. The lithographymirror of claim 1, wherein said insulating sublayer comprises adielectric material.
 8. The lithography mirror of claim 1, wherein saidreflective layer comprises moly-silicon multilayer.
 9. The lithographymirror of claim 1, wherein said contacts are coupled to edges of saidresistive layer such that said contacts are diametrically opposed to oneanother.
 10. The lithography mirror of claim 1, wherein at least one ofsaid resistive layer, said insulating sublayer, and said polished layeris formed to a peripheral edge of said substrate.
 11. The lithographymirror of claim 3, wherein said variable power supply varies anadditional heat load inversely to the actinic heat load.
 12. Thelithography mirror of claim 1, wherein said resistive layer comprisesnichrome.
 13. The lithography mirror of claim 1, wherein said resistivelayer comprises carbon.
 14. The lithography mirror of claim 1, whereinsaid resistive layer comprises ceramic and metal.
 15. The lithographymirror of claim 1, further comprising: feedforward means for measuringthe actinic heat load.
 16. The lithography mirror of claim 15, furthercomprising feedback means for regulating an additional heat load appliedto said contacts.
 17. The lithography mirror of claim 15, wherein saidfeedforward means is a heat flux sensor that measures the actinic heatload.
 18. The lithography mirror of claim 15, wherein said feedforwardmeans comprises at least one infrared detector to monitor a frontsurface of the lithography mirror.
 19. The lithography mirror of claim1, wherein said resistive layer has a temperature coefficient ofresistance (TCR) high enough to provide temperature feedback.
 20. Thelithography mirror of claim 1, further comprising feedback means forregulating an additional heat load applied to said contacts.
 21. Thelithography mirror of claim 20, wherein said feedback means comprises atleast one thermocouple fixed onto a front surface of the lithographymirror that regulates an additional heat load in said resistive layer.22. The lithography mirror of claim 20, wherein said feedback means isphotographically formed onto said insulating sublayer.
 23. Thelithography mirror of claim 1, wherein said resistive layer is producedby doping a semiconductor film such that electrical conductivity of thecenter of said resistive film varies from electrical conductivity of theperiphery of said electrically resistive film.
 24. The lithographymirror of claim 1, wherein the thickness of the center of said resistivelayer varies from the thickness at the periphery of said resistivelayer.
 25. The lithography mirror of claim 1, wherein the thickness ofsaid resistive layer, said insulating sublayer, said polished layer, andsaid reflective layer is less than or equal to one micron.
 26. Thelithography mirror of claim 1, wherein said contacts are comprised ofconductive material.
 27. A lithography mirror for use in a lithographysystem that enables management of actinic heat load, comprising: asubstrate; a heated annular zone formed on said substrate; and a heatedoptical aperture zone formed on said heated annular zone.
 28. Thelithography mirror of claim 27, wherein each zone comprises: a resistivelayer; and a wiring layer formed on said resistive layer.
 29. Thelithography mirror of claim 28, wherein said wiring layer includes aninsulating sublayer and contacts for coupling to a power supply.
 30. Thelithography mirror of claim 29 wherein said insulating sublayercomprises a polymer.
 31. The lithography mirror of claim 29, whereinsaid insulating sublayer comprises a nonconductive material.
 32. Thelithography mirror of claim 29, wherein said insulating sublayercomprises silicon dioxide.
 33. The lithography mirror of claim 29,wherein said insulating sublayer comprises a dielectric material. 34.The lithography mirror of claim 29, further comprising a variableresistor coupled to said contacts to vary an additional heat loadinversely to the actinic heat load.
 35. The lithography mirror of claim29, wherein said contacts in said wiring layer in said heated opticalaperture zone are diametrically opposed to each other.
 36. Thelithography mirror of claim 29, wherein said contacts in said wiringlayer in said heated annular zone are positioned concentrically suchthat the flow of electricity from a first of said contacts to a secondof said contacts produces a relatively annular and uniform heatingpattern.
 37. The lithography mirror of claim 31, wherein an additionalheat load is inversely modulated in said optical aperture zone accordingto an amount of actinic heat in said optical aperture zone andconstantly applied in said annular zone to mitigate a cold edge effect.38. The lithography mirror of claim 31, wherein said resistive layercomprises nichrome.
 39. The lithography mirror of claim 31, wherein saidresistive layer comprises carbon.
 40. The lithography mirror of claim31, wherein said resistive layer comprises ceramic and metal.
 41. Thelithography mirror of claim 28, further comprising: feedforward meansfor measuring the actinic heat load.
 42. The lithography mirror of claim28, further comprising feedback means for regulating an additional heatload applied via said contacts.
 43. The lithography mirror of claim 28,wherein said resistive layer has a temperature coefficient of resistance(TCR) high enough to provide temperature feedback.
 44. The lithographymirror of claim 41, further comprising feedback means for regulating anadditional heat load applied via said contacts.
 45. The lithographymirror of claim 41, wherein said feedforward means is a heat flux sensorthat measures said actinic heat load.
 46. The lithography mirror ofclaim 42, wherein said feedback means comprises at least onethermocouple on a front surface of the lithography mirror that regulatessaid additional heat load in said resistive layer.
 47. The lithographymirror of claim 42, wherein said feedback means is photographicallyformed onto said insulating sublayer.
 48. The lithography mirror ofclaim 41, wherein said feedforward means comprises at least one of aninfrared detector and a pyroelectric detector to monitor a front surfaceof the lithography mirror.
 49. The lithography mirror of claim 29,wherein said resistive layer is produced by doping a semiconductor filmsuch that electrical conductivity of the center of said resistive filmvaries from electrical conductivity of the periphery of saidelectrically resistive film.
 50. The lithography mirror of claim 29,wherein the thickness of the center of said resistive layer varies fromthe thickness at the periphery of said resistive layer.
 51. Thelithography mirror of claim 29, wherein the thickness of said layers isless than or equal to one micron.
 52. The lithography mirror of claim30, wherein said contacts in each zone are comprised of conductivematerial.
 53. A method for manufacturing a lithography mirror, having asubstrate, for use in a lithography system to manage actinic heat loadon the substrate, comprising: (a) forming a resistive layer on thesubstrate; (b) forming a wiring layer on the resistive layer, whereinsaid wiring layer includes an insulating sublayer and contacts forcoupling to a power supply. (c) forming a polished layer on theinsulating sublayer; and (d) forming a reflective layer on the polishedlayer.
 54. The method of claim 53, wherein the polished layer is formedon the insulating sublayer and the contacts.
 55. The method of claim 53,further comprising the step of: (e) forming at least one of theresistive layer, the insulating sublayer, the polished layer, and thereflective layer to a peripheral edge of the substrate.
 56. The methodof claim 53, further comprising the step of coupling the contacts toedges of the resistive layer such that the contacts are diametricallyopposed to one another.
 57. The method of claim 53, further comprisingthe step of providing feedforward means for measuring the actinic heatload.
 58. The method of claim 57, further comprising the step ofproviding feedback means for regulating an additional heat load appliedvia the contacts.
 59. The method of claim 58, further comprising thestep of forming the feedback means onto the insulating sublayer.
 60. Themethod of claim 58, further comprising the step of photographicallyforming the feedback means onto the insulating sublayer.
 61. The methodof claim 53, further comprising the step of coupling at least onethermocouple to the front surface of the lithography mirror forregulating an additional heat load in the resistive layer.
 62. Themethod of claim 53, further comprising the step of utilizing infrareddetectors and/or pyroelectric detectors to monitor the front surface ofthe lithography mirror.
 63. The method of claim 53, further comprisingthe step of providing feedback means for regulating an additional heatload applied via the contacts.
 64. The method of claim 53, wherein step(a) comprises the steps of: (1) forming a resistive layer on thesubstrate; and (2) doping the resistive layer such that conductivity ofthe center of the resistive layer varies from electrical conductivity ofthe periphery of the resistive layer.
 65. The method of claim 53,further comprising the step of varying thickness of at least one layerof the lithography mirror.
 66. The method of claim 53, wherein step (a)comprises the step of forming a resistive layer on the substrate,wherein the resistive layer has a temperature coefficient of resistance(TCR) high enough to provide temperature feedback.
 67. In a lithographysystem, a method for managing actinic heat load on a lithography mirrorhaving a substrate, a resistive layer formed on the substrate, a wiringlayer formed on the resistive layer, a polished layer formed on aninsulating sublayer, and a reflective layer formed on the polishedlayer, comprising: (a) coupling a power supply to contacts on the wiringlayer; (b) monitoring a temperature of the lithography mirror; and (c)adjusting the power supply to maintain a desired temperature of thelithography mirror.
 68. The method of claim 67, wherein said step (b)comprises the step of measuring the actinic heat load on the lithographymirror with a feedforward means.
 69. The method of claim 67, furthercomprising the step of regulating an additional heat load on thelithography mirror with at least one thermocouple.
 70. The method ofclaim 67, further comprising the step of utilizing infrared detectorsand/or pyroelectric monitors to monitor a front surface of thelithography mirror.
 71. The method of claim 67, further comprising thestep of: (d) maintaining time-constant temperature in the mirror byutilizing the heaterstat control method.
 72. A lithography mirror foruse in a lithography system that enables management of actinic heatload, comprising: a substrate; a wiring layer formed on said substratewherein said wiring layer includes an insulating sublayer and contactsfor coupling to a power supply; a resistive layer formed on said wiringlayer; a polished layer formed on said resistive layer; and a reflectivelayer formed on said polished layer.
 73. A method for manufacturing alithography mirror, having a substrate, for use in a lithography systemto manage actinic heat load on the substrate, comprising: (a) forming awiring layer on the substrate, wherein the wiring layer includes aninsulating sublayer and contacts for coupling to a power supply; (b)forming a resistive layer on the wiring layer; (c) forming a polishedlayer on the resistive layer; and (d) forming a reflective layer on thepolished layer.